1. Field of the Invention
The present invention relates to a radiographic phase-contrast imaging apparatus using gratings.
2. Description of the Related Art
X-rays have a nature that they attenuate depending on the atomic number of an element forming a substance and the density and thickness of the substance. Because of this nature, X-rays are used as a probe to investigate the interior of a subject. Imaging systems using X-rays have widely been used in the fields of medical diagnosis, nondestructive testing, etc.
With a typical X-ray imaging system, a subject is placed between an X-ray source, which emits an X-ray, and an X-ray image detector, which detects an X-ray image, to take a transmission image of the subject. In this case, each X-ray emitted from the X-ray source toward the X-ray image detector attenuates (is absorbed) by an amount depending on differences of characteristics (such as the atomic number, density and thickness) of substances forming the subject present in the path from the X-ray source to the X-ray image detector before the X-ray enters the X-ray image detector. As a result, an X-ray transmission image of the subject is detected and imaged by the X-ray image detector. As examples of such an X-ray image detector, a combination of an X-ray intensifying screen and a film, a photostimulable phosphor (storage phosphor), and a flat panel detector (FPD) using a semiconductor circuit are widely used.
However, the smaller the atomic number of an element forming a substance, the lower the X-ray absorbing capability of the substance. Therefore, there is only a small difference of the X-ray absorbing capability between soft biological tissues or soft materials, and it is difficult to obtain a sufficient contrast of the image as the X-ray transmission image. For example, articular cartilages forming a joint of a human body and synovial fluids around the cartilages are composed mostly of water, and therefore there is only a small difference of the X-ray absorption therebetween and it is difficult to obtain an image with sufficient contrast.
In recent years, X-ray phase-contrast imaging for obtaining a phase contrast image based on phase variation of X-rays due to differences between refractive indexes of a subject, in place of the intensity variation of X-rays due to differences between absorption coefficients of the subject, have been studied. With this X-ray phase-contrast imaging using the phase difference, a high contrast image can be obtained even in the case where the subject is a substance having low X-ray absorbing capability.
As an example of such an X-ray phase-contrast imaging system, a radiographic phase-contrast imaging apparatus has been proposed in International Patent Publication No. 2008/102654 and Japanese Unexamined Patent Publication No. 2010-190777 (hereinafter, Patent Documents 1 and 2), wherein two gratings including a first grating and a second grating are arranged parallel to each other at a predetermined interval, a self image of the first grating is formed at the position of the second grating due to the Talbot interference effect of the first grating, and the intensity of this self image is modulated with the second grating to provide a radiographic phase-contrast image.
With the radiographic phase-contrast imaging apparatus disclosed in Patent Documents 1 and 2, a fringe scanning method is performed, where the second grating is positioned almost parallel to the plane of the first grating, and the first grating or the second grating is relatively translated in a direction that is almost orthogonal to the direction of the grating by a predetermined amount that is smaller than the grating pitch. By performing an imaging operation each time the grating is translated, a plurality of images are taken. Based on these images, an amount of phase variation (phase shift differential) of an X-ray generated by interaction with the subject is obtained. Then, based on this phase shift differential, a phase-contrast image of the subject can be obtained.